Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A commercial building has a 400A, 480Y/277V, 3-phase, 4-wire service. The service entrance conductors are 500 kcmil copper. What is the minimum size required for the service grounded conductor?
- A4/0 AWG copper
- B3/0 AWG copper
- C2/0 AWG copper
- D1/0 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: C. 2/0 AWG copper
According to NEC 250.24(D)(1) and Table 250.102(C)(1), the service grounded conductor shall not be smaller than the minimum size required for the grounding electrode conductor specified in Table 250.66. It also states it shall not be smaller than required by 250.102(C)(1). For 500 kcmil copper ungrounded conductors, Table 250.102(C)(1) requires a 2/0 AWG copper conductor as the minimum size for the main bonding jumper and by extension, the grounded conductor.
Why the other options are wrong
- A. This size is larger than the minimum required but could be used.
- B. This size is larger than the minimum required but could be used.
- D. This size is too small for 500 kcmil copper service conductors.
Service Grounded Conductor Sizing (250.24)
The service grounded conductor (neutral) is sized to carry the maximum unbalanced current and to meet minimum equipment grounding conductor requirements.
- Sized per NEC 220.61 for maximum unbalanced load.
- Minimum size per 250.24(D) not less than required by 250.102(C)(1) for the main bonding jumper.
- Also not smaller than required by 250.66 for the grounding electrode conductor.
Memory trick: Neutral needs balance and ground protection.